During the lifetime of a patient, it may be necessary to perform a joint replacement procedure to address damage to the joint due to, for example, disease or trauma. The joint replacement procedure may involve the use of a prosthesis which is implanted into at least one of the patient's bones. In the case of a hip replacement procedure, a femoral prosthesis is implanted into the patient's thigh bone or femur. The femoral prosthesis had been typically constructed as a one-piece structure having an upper portion that includes a spherically-shaped head which bears against the patient's pelvis or acetabulum, along with an elongated intramedullary stem which is utilized to secure the femoral component to the femur. In order to secure this prosthesis to the patient's femur, the medullary canal of the patient's femur is first surgically prepared (e.g. reamed and/or broached) such that the intramedullary stem of the femoral prosthesis may be subsequently implanted therein. The femoral prosthesis may be press fit into the medullary canal or, in the alternative, bone cement may be utilized to secure the femoral prosthesis within the medullary canal.
During performance of a joint replacement procedure, it is generally desirable to provide the surgeon with a certain degree of flexibility in the selection of a prosthesis. In particular, the anatomy of the bone into which the prosthesis is to be implanted may vary somewhat from patient to patient. For example, in the case of a femoral prosthesis, the patient's femur may be relatively long or relatively short thereby requiring use of a femoral prosthesis which includes a stem that is relatively long or short, respectively. Moreover, in certain cases, such as when use of a relatively long stem length is required, the stem must also be bowed in order to conform to the anatomy of the patient's femur. The trochanteric region of the femur may require a specially sized proximal portion of the prosthesis or a particular angle of the proximal portion to mate with the articulations of the hip joint.
Such a need for prostheses of varying shapes and sizes created a number of problems in regard to use of a one-piece prosthesis. As a result of these problems, the modular prosthesis was developed. As the name implies, a modular prosthesis is constructed in modular form so that the individual elements or features of the prosthesis can be selected to fit the needs of a given patient's anatomy. Examples of certain modular prostheses are found in U.S. Pat. No. 6,706,072 (the '072 Patent), issued to DePuy Orthopaedics, Inc., including the two prosthesis illustrated in FIGS. 1-2. In general, the modular femoral prosthesis, such as the prosthesis 10 of FIG. 1, includes a proximal neck component 12 that includes a body 24 that terminates in a trunnion 26 onto which a variety of head bearing components (not shown) that mate with the patient's natural acetabulum or to a prosthetic socket.
The neck component 12 fits within a sleeve 14 that is configured to fit within a surgically prepared end of the femur. In a joint replacement procedure, the head of the natural femur is removed to form a flat surface across the trochanters of the femur. The medullary canal of the femur is reamed and/or broached and a cavity is formed in the proximal prepared end of the bone to receive the sleeve 14. The sleeve component 14 may be provided in carrying angles and lengths to fit the needs of the patient's joint anatomy. A distal stem component 18 is provided that is implanted within the prepared medullary canal. As indicated above, the stem component 18 may be provided in various sizes and curvatures to accommodate the anatomy of the particular femur.
All of the components of the modular prosthesis are configured to be rigidly engaged when the prosthesis 10 is implanted. Thus, the prosthesis 10 described in the '072 Patent includes a tapered shank 13 projecting from the body 24 of the proximal neck component 12. The shank 13 extends into a tapered bore 30 of the sleeve 14 in a press-fit engagement. The two tapered elements (shank 13 and bore 30) may define a Morse taper for rigid fixation between the sleeve and neck component. The shank 13 also defines a tapered bore 22 for a press-fit engagement with a tapered post 19 of the distal stem component 18. The end of the post 19 may be provided with threads 28 to be threaded into mating threads 29 in the shank 13 of the proximal neck component 12.
A modified prosthesis 10′ is shown in FIG. 2. This modified prosthesis includes a sleeve 14 that is substantially the same as the sleeve shown in FIG. 1. The proximal neck component 12′ is similar to the neck component 12 in that includes a tapered shank 13′ for press-fit engagement within the mating bore of the sleeve 14. However, the bore 22′ of the neck component 12′ extends entirely through the component. The distal stem component 18′ of FIG. 2 is similar to the stem component 18 of FIG. 1, except that the tapered post 19′ is longer and terminates in a threaded end 28′ that is configured to be engaged by a threaded nut or cap 29′.
With either prosthesis 10 or 10′, the head of the femur is prepared as described above and the sleeve 14 pressed into the prepared proximal end. Bone cement may be used to help fix the sleeve 14 within the bone. The distal stem component 18/18′ is introduced through the sleeve bore 30 into the prepared medullary canal of the femur. The proximal neck component 12/12′ is then positioned with the sleeve bore 30 and advanced into the bore so that the tapered bore 22/22′ fits over the tapered post 19/19′ of the distal stem component. The threaded end 28/28′ is threaded into the appropriate mating component for the two prostheses 10/10′. One concern that has arisen with modular prostheses is the locking of the components relative to one another. The fixation of the distal stem component 18/18′ to the proximal neck component 12/12′ and of the neck component to the sleeve 14 in the prostheses 10/12′ of FIGS. 1-2 address this concern. This fixation among the components allows the components to adequately absorb the axial functional loads exerted on the prosthesis without appreciable degradation or even breach of the mechanical integrity prosthesis. Ideally, the functional loads on these prostheses 10/10′ increase or improve the self-locking attributes of the devices.
In some joint replacements a revision of the prosthesis may be necessary. For instance, in some cases, a distal stem component is implanted that has a version that is determined to be less than optimal for the anatomy of the patient's femur. In other cases, the proximal neck component or distal stem requires replacement due to wear or damage, or because of changes in the patient's joint anatomy that can occur over time. At any rate, the otherwise beneficial self-locking characteristics of the modular prostheses 10/10′ become a disadvantage in a revision procedure where it is necessary to separate the neck and stem components from the stable well-fixed sleeve.
The current method of removing the neck and stem components from the sleeve is to use a bone chisel to wedge between the proximal surface 16 of the sleeve 14 and the opposing surface 25 of the neck component 12/12′ (FIG. 1). A bone chisel is initially pushed into the space between the two surfaces 16, 25 to incrementally disassociate the components from the sleeve. When the gap is large enough, a second bone chisel is introduced into the space, and so on with multiple chisels until the gap is large enough to accept an impaction tool. The distal surface 25 of the neck component 12/12′ is impacted until it is dislodged from the sleeve.
This technique suffers from a few problems. Foremost is the inherent difficulties associated with using a bone chisel to wedge fixed components apart. The chisel may slip out of place and damage surrounding tissue or cut through the surgeon's gloves. In addition, the use of an impaction tool to separate the components from the sleeve tends to produce an axial load on the sleeve that can tend to disassociate the sleeve from the proximal end of the femur.
Consequently, there is a need for a tool that can be used to remove the neck and stem components of a modular prosthesis from the sleeve, while keeping the sleeve intact within the prepared bone.